In the manufacture of hot-pressed or sintered metal carbides and borides from powders, a powder, such as TiB.sub.2, is compressed in a mold at an elevated temperature to sinter the particles of the powder together. Often, these sintered shapes are highly stressed and have poor resistance to mechanical and thermal shock. This is believed to be due in large part to the thermal anisotropy of many of these materials, such as TiB.sub.2, and the large grain or particle size in the sintered shapes. Even though finely divided powders may be employed to form the sintered shapes, the temperatures and pressures in which sintered shapes are commonly formed contribute to grain growth, resulting in sintered shapes with undesirably large grain size.
The effect of large grain size can be shown by the well-known Griffith-Irwin fracture equation. According to this equation, the strength (S) of ceramics is a function of the critical flaw size (c), and the stress intensity factory (Kc) as follows: ##EQU1## wherein Y is a geometrical factor. Critical flaws are usually related to grain size, i.e. large grains will geometrically accommodate larger defects in the sintered shape. Therefore, the critical flaw size of sintered bodies with large grain size will generally be greater,
and the strength will be smaller, than for sintered bodies with a smaller grain size. In addition, for thermally anisotropic materials, like TiB.sub.2, the tendency for cracks developing in the grains or crystallites themselves or in grain boundaries increases with the grain size. Thus, in order to maximize the strength of sintered bodies, and minimize the tendency of crack formation in the crystallites and grain boundaries, it is desirable to maintain as low a grain size as possible.
An additional problem is that the network of cracks formed in large grained sintered structures affect not only the mechanical properties, such as strength, Young's modulus, fracture toughness, and the like, but also increase the permeability of the structure to molten metals and gasses, which is often undesirable for sintered shapes used in refractory applications.
Because of the difficulty in forming small grain sintered bodies, due to grain growth during formation of the body, there is a need for a method wherein sintered bodies can be formed without the formation of large grains.
U.S. Pat. No. 4,308,114, issued to Das et al., discloses the manufacture of titanium diboride electrodes for Hall cells using a small amount of carbonaceous material to act as a scavenger during the sintering of the shape.
U.S. Pat. No. 3,661,736, issued to Holliday, discloses the manufacture of electrodes for Hall cells comprising graphite and 2 to 50 volume percent refractory hard metal.